Pressing a wet paper web in a press section at elevated temperature has been shown to result in a number of beneficial effects. The technology, known as impulse technology, impulse drying or impulse pressing, was first described in SE-7803672-0, corresponding to U.S. Pat. No. 4,324,613, which discloses a roll press. The top pressure utilized in the process described therein lies within the range of 3-8 MPa and the surface temperatures between 150.degree. C. and 350.degree. C. for a cylinder in a press nip of conventional type. The dwell time during which a given region of the web is within the nip in such a roll press, however, is only a few milliseconds, which is too short a time to be able to take full advantage of the favorable effects of pressing at high temperature. It has therefore been proposed subsequently to utilize impulse drying in a heated shoe press in which the nip is extended so that the dwell time for the heat treatment is greatly extended.
In impulse drying, high temperature and high pressure are combined for a brief period of time in order to give extremely high heat flows to the web. The envelope surface of a steel or cast iron roll has maximum heat flow in the range of 2-8 MW/m.sup.2, which results in extremely high dewatering speeds. The mechanism forming the basis of these high dewatering speeds, with consequent high dry solids content, has not yet been fully discerned. One theory presented is that, through its expansion, the steam which is developed in the vicinity of the hot surface of the heat-transfer element nearest the fibrous web helps to displace water remaining in the fibrous web into the felt that is in contact with the fibrous web. Another theory is that high dry solids content is achieved by virtue of the reduced viscosity of the water caused by the high temperature, in combination with water at high temperature (&gt;100.degree. C.) being quickly vaporized when the web leaves the nip and the pressure thus drops to atmospheric pressure.
According to the '613 patent, a burner supplies heat to one roll in a press with two rotating rolls. The heat is supplied to the envelope surface of the roll immediately before the press nip. The patent specification states that the roll may have a surface layer with low thermal conductivity, thereby enabling it to maintain a high temperature.
U.S. Pat. No. 4,738,752 describes an extended, heated press nip where the fibrous web encounters a hot surface formed by a rotatable press roll or a metal belt passing in a loop around a plurality of guide rolls. The press roll or belt is heated by a heat source. The press roll may have a first coaxial layer and a second coaxial layer extending around the first layer and having a coefficient of thermal conductivity that is greater than that of the first layer. The first layer may consist of ceramic, whereas the second, outer layer consists of metal and has a thickness of 0.0127-1.27 cm. The layers are in intimate contact with each other and, together, form a unitary body.
When a roll of the described known types is heated from the outside, the outer layer will be hotter than the layer or layers disposed inside the outer layer which is rigidly joined to the next inner layer. The outer layer will therefore expand more than the inner layer, thereby causing stresses to arise between the two layers. Even if the roll is homogenous, the same differences will arise in expansion and stresses between the outer and inner parts. To reduce the risk of damage in the roll as a result of such stress differences, the initial heating of the roll must take place slowly. Another problem is that it is difficult to maintain the geometric shape of the roll across the machine direction, because of difficulties in maintaining the same temperature along the envelope surface of the roll and at the end walls. Considerable stresses also arise between these construction elements since they cannot expand freely independently of each other, and the envelope surface can become bowed outwardly or inwardly across the machine direction. Since the inner layer or part of the roll will also absorb some of the thermal energy supplied, the heating costs will be high.
One drawback with the use of a metal belt as a heat-transfer means is that it must be arranged in a loop with at least two rolls, thus constituting a space-consuming configuration. To allow the surface of the belt to be cleaned with a doctor, a counter roll must be arranged inside the loop of the belt in front of the doctor. Another drawback is that the belt cannot usually be coated with layers to achieve certain release properties and certain thermal conductivity properties.
A number of the problems discussed above exist also in roll presses and calendars that supply heat to the web.